GBS Elektronik MCA 166 - Manual (Page 21)

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21
Shaping time
Use the space bar to toggle between the two time constants 1 and 2 microseconds. The
shaping time is the time constant of the RC in the pulse shaping network. The pulse
FWHM time is about 3.5 times the shaping time, the total pulse time (from 1% peak
amplitude to 1% peak amplitude) may be as long as 10 times the shaping time. Use 2
microsec for low countrate measurements with a HPGe detector where the best
resolution is needed and 1 microsec in all other cases.
Polarity
Use the spacebar to toggle between amplifier input pulses with negative or positive input
polarity. Remember: Wrong polarity is a common reason for measuring strange spectra or
getting no signal at all.
PUR
Use the spacebar to turn the pile up rejector (PUR) "on" or "off". The pile up rejector
should cancel pulses which are very close together in time in order to avoid registering
them as one pulse with the energy summed. The corresponding dead time is accounted
for by the dead time correction scheme.
Do visual PZC adjustment
Press Enter to access the graphical screen which visualizes the pole zero adjustment. For
adjusting PZC a count rate of 1000 - 10000 should be optimum, (300 - 30000 may still
work) and the majority of these pulses should fill the measurement range at least to the
half. The bar indicates the difference between the voltage at the ADC 11 µs after the
pulse peak (22 µs for 2 µs shaping time) and 100 µs after the pulse peak. This voltage
should be adjusted to zero. A wrong PZC setting may lead to a degraded resolution, in
particular at high countrates.
It is also possible to observe the amplifier signal at the amplifier test output with a scope
using a high impedance, low capacitance probe and to adjust PZC.
3.2.3.3 Stabilization setup
This function is mostly useful for scintillation detectors as NaI. The pulse amplitude of
these detectors depends not only on photon energy, but is also sensitive to
photomultiplier voltage and crystal temperature. This drift can be compensated by
regulating the fine gain of the amplifier to keep the position of a reference peak. The
reference peak has to be marked as a ROI. Usual is to use the alpha peak of an Am241 -
source. Some detectors have for this purpose a built-in Am241 source. In enrichment
measurements, it is also possible to stabilize on the 186 keV peak of U235.
To activate stabilization, turn on stabilization by toggling the stabilization on/off setting
with the spacebar. You are then asked to accept or to change the stabilization ROI
settings.
3.2.3.4 Preamplifier power setup
Allows to turn certain preamplifier supply voltages off to extend battery life by about 10%.
Use the spacebar to turn a preamplifier supply voltage off in case the used preamplifier